In mathematics, particularly algebraic topology, cohomotopy sets are particular contravariant functors from the category of pointed topological spaces and basepoint-preserving continuous maps to the category of sets and functions. They are dual to the homotopy groups, but less studied.

Overview

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The p-th cohomotopy set of a pointed topological space X is defined by

 

the set of pointed homotopy classes of continuous mappings from   to the p-sphere  .[1]

For p = 1 this set has an abelian group structure, and is called the Bruschlinsky group. Provided   is a CW-complex, it is isomorphic to the first cohomology group  , since the circle   is an Eilenberg–MacLane space of type  .

A theorem of Heinz Hopf states that if   is a CW-complex of dimension at most p, then   is in bijection with the p-th cohomology group  .

The set   also has a natural group structure if   is a suspension  , such as a sphere   for  .

If X is not homotopy equivalent to a CW-complex, then   might not be isomorphic to  . A counterexample is given by the Warsaw circle, whose first cohomology group vanishes, but admits a map to   which is not homotopic to a constant map.[2]

Properties

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Some basic facts about cohomotopy sets, some more obvious than others:

  •   for all p and q.
  • For   and  , the group   is equal to  . (To prove this result, Lev Pontryagin developed the concept of framed cobordism.)
  • If   has   for all x, then  , and the homotopy is smooth if f and g are.
  • For   a compact smooth manifold,   is isomorphic to the set of homotopy classes of smooth maps  ; in this case, every continuous map can be uniformly approximated by a smooth map and any homotopic smooth maps will be smoothly homotopic.
  • If   is an  -manifold, then   for  .
  • If   is an  -manifold with boundary, the set   is canonically in bijection with the set of cobordism classes of codimension-p framed submanifolds of the interior  .
  • The stable cohomotopy group of   is the colimit
 
which is an abelian group.

History

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Cohomotopy sets were introduced by Karol Borsuk in 1936.[3] A systematic examination was given by Edwin Spanier in 1949.[4] The stable cohomotopy groups were defined by Franklin P. Peterson in 1956.[5]

References

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  1. ^ "Cohomotopy_group", Encyclopedia of Mathematics, EMS Press, 2001 [1994]
  2. ^ "The Polish Circle and some of its unusual properties". Math 205B-2012 Lecture Notes, University of California Riverside. Retrieved November 16, 2023. See also the accompanying diagram "Constructions on the Polish Circle"
  3. ^ K. Borsuk, Sur les groupes des classes de transformations continues, Comptes Rendue de Academie de Science. Paris 202 (1936), no. 1400-1403, 2
  4. ^ E. Spanier, Borsuk’s cohomotopy groups, Annals of Mathematics. Second Series 50 (1949), 203–245. MR 29170 https://doi.org/10.2307/1969362 https://www.jstor.org/stable/1969362
  5. ^ F.P. Peterson, Generalized cohomotopy groups, American Journal of Mathematics 78 (1956), 259–281. MR 0084136